Fix test failure

Author: YingboMa

src/systems/abstractsystem.jl

@@ -527,7 +527,10 @@ function structural_simplify(sys::AbstractSystem)
     if sys isa ODESystem
         sys = dae_index_lowering(sys)
     end
-    sys = sort_states(tearing(sys))
+    sys = tearing(sys)
+    if sys isa ODESystem
+        sys = sort_states(sys)
+    end
     fullstates = [map(eq->eq.lhs, observed(sys)); states(sys)]
     @set! sys.observed = topsort_equations(observed(sys), fullstates)
     return sys